The present invention relates to processes for the preparation of graft copolymers. These processes involve the use of polar solvents in graft polymer extrusion reactions. This invention further relates to processes for preparing graft polymers useful as dispersant viscosity index improvers.
Solution grafting is a well known method for producing polymers. Typically, various reagents are introduced, either neat or as solutions in various solvents, into one or more reaction vessels containing solvent. The desired polymer product must sometimes then be separated from the reaction solvents and/or impurities by appropriate purification steps. Thus, these methods are limited by factors such as: practicality, availability and cost of solvents; practicality and economics of required purification procedures; and concerns surrounding generation and disposal of waste products associated with the processes.
Even when the reaction medium makes up part of the desired end product, solution grafting may be undesirable in terms of costs and logistics of transporting and storing the end product compared to a more concentrated polymer product. For example, some dispersant viscosity index improvers (DVII""s) are made by a solution grafting process where a base oil is the solvent (See for example, U.S. Pat. No. 5,523,008). This is particularly acceptable where the DVII in base oil will ultimately comprise a lubricating oil formulation. In certain instances, however, it would be more economical and safer to transport and store a more concentrated form of the DVII for later blending with the base oil.
Current methods of producing polymer products for lubrication applications require that the reaction polymer product be further processed in order to achieve the desired physical characteristics. For example, to achieve a desired shear stability index (SSI), a measure of potential for in-service viscosity loss, the polymer product must be subjected to homogenization (i.e. mechanical shearing) in order to create a polymer product with uniform and consistent viscosity characteristics. The presence of undesirable reaction by-products is responsible, in part, for the necessity of such further processing. Thus, reducing the formation of reaction side products may also reduce or eliminate the necessity for downstream homogenization in order to achieve a polymer product with the desired finished characteristics.
One such undesirable side polymerization process that may occur is coupling of the polymer starting material with itself. This chain extension process leads to larger chain polymers that ultimately affect the purity of the graft polymer product resulting from the desired polymerization reaction.
Another undesirable side polymerization process that may occur in solution grafting processes is reaction between the solvent and the monomer. For instance, in solution grafting where base oil is the solvent, this type of side polymerization process results in the production of monomer-grafted base oil polymers. These by-products may contaminate the desired graft polymer and thus may be introduced into the lubrication oil formulations with the desired graft polymer product. The presence of the monomer-grafted base oil polymers in the lubrication oil formulations detrimentally affects seal compatibility in engines utilizing such formulations.
Grafted polymers made by solution processes include those described in U.S. Pat. No. 5,523,008. U.S. Pat. No. 5,523,008 (herein incorporated by reference in its entirety) discloses novel grafted polymers comprising polyolefin backbones grafted with ethylenically unsaturated nitrogen or oxygen-containing monomers. Additionally, dispersant/viscosity index improvers (xe2x80x9cDVII""sxe2x80x9d) and lubricating oil compositions comprising the grafted polymer products are disclosed. Furthermore, methods for manufacturing such grafted polymers, DVII""s and lubricating oil compositions are disclosed therein. The grafted polymers described in this patent are shown to possess desirable properties and characteristics that are advantageous for use as dispersant/viscosity index improvers and as dispersants.
Another example of grafted polymers made by solution processes include those described in U.S. Pat. No. 5,298,565. U.S. Pat. No. 5,298,565 describes graft copolymers prepared by grafting, onto a polymeric substantially saturated hydrocarbon backbone, a free-radical polymerizable vinyl nitrogen monomer. While the disclosure teaches a certain stoichiometry of starting reagents to make such graft polymers, it is unclear what actual proportion of grafted monomer is present in the polymer product. Additionally, this patent teaches preparation of the graft copolymers by a solution grafting process using aliphatic hydrocarbon-substituted aromatic solvents. The disclosure also states that these graft copolymers may be prepared in different types of reactors, including extruders and kettle-type reactors (col. 11, lines 50-52).
Extrusion reactions are an alternative to the solution grafting processes described above. U.S. Pat. No. 5,424,367 describes processes for carrying out multiple sequential chemical reactions on polymeric feedstock carried out in an extruder with multiple reaction zones. The key feature of the process is the removal of impurities from one reaction zone before a subsequent reaction occurs in a subsequent reaction zone in the extruder. In one embodiment of this process, feed of water upstream of the first reaction zone leads to less colored products. An additional embodiment described in this application is a process for the production of grafted polymers in an extruder reactor by feeding water prior to the grafting reaction zone. It is understood that the water is not incorporated into the polymer when using the procedure outlined for introduction of the water into the extruder.
Existing extrusion processes, however, still suffer from some limitations. For instance, not all types of reactants are amenable for use in existing extrusion methods because of their inability to be introduced and mixed into the reaction process such that a sufficiently uniform grafted product results.
Thus, a need still exists for improved methods of making grafted copolymer products. Ideally, such methods should be more efficient, reduce undesired by-product formation and be less costly to operate.
This invention provides a method of making a graft copolymer, comprising the steps of:
(a) providing (i) an ethylene/propylene (EP) copolymer or other graftable polymer, (ii) an ethylenically unsaturated sulfur-, nitrogen- and/or oxygen-containing monomer, and (iii) an amount of an initiator sufficient to graft said monomer and EP copolymer or other graftable polymer;
(b) introducing said EP copolymer or other graftable polymer into a melt-blending apparatus, preferably an extruder;
(c) introducing said monomer into the melt-blending apparatus, preferably the extruder;
(d) introducing said initiator into the melt-blending apparatus, preferably the extruder; wherein at least one of the above-identified reactants in (a) is introduced into the melt-blending apparatus in the presence of at least either a polar or a non-polar solvent; and
(e) reacting said EP copolymer or other graftable polymer, monomer and initiator by operating the melt-blending apparatus, preferably the extruder, thereby forming the graft copolymer.
This method can be used for producing a graft polymer containing at least 8 moles of N-vinylimidazole (VIMA) per mole of EP copolymer backbone. The graft polymer can alternatively be defined as containing more than about 0.7% by weight of VIMA grafts on EP copolymer backbone. The polyolefin has a weight average molecular weight of from about 20,000 to about 500,000 and a polydispersity of less than about 10. This method has several advantages over existing methods of making such graft polymers. The extrusion process with reactant introduction in a solvent allows for greater reaction efficiency in utilization of raw materials. The extrusion process dispenses with the necessity for high solvent volumes as compared to solution grafting processes. This results in lower reaction costs associated with the reduced requirements for solvent, not having to remove the solvent from the graft polymer product and savings associated with ease of transport of the more concentrated form of grafted polymer product that results from these processes.
The extrusion processes of this invention, which do not utilize the solvent volumes typically used in solution grafting, minimize or eliminate production of undesirable solvent grafted by-products. These by-products are produced by the side reaction of reactants with the solvent during the grafting reaction process.
The use of polar solvents in these processes provides improved mixing of the reactants, which aids in the suppression of undesirable side polymerization reactions, and thus improved reaction efficiency. They also participate to some degree in control of reaction exotherms through their volatility, i.e., some of the heat generated in the reaction is dissipated by xe2x80x9cflashingxe2x80x9d of the solvent from liquid to a gas that is vented away. These processes also allow for grafting with reactive chemical intermediates that heretofore could not be utilized in an extruder.
This invention can also minimize undesired side reactions by utilizing saturated polymer starting materials, such as ethylene/propylene (EP) copolymer. Such saturated polymers have a reduced propensity to undergo coupling side polymerization reactions as compared to polymers possessing sites of unsaturation.
The methods of this invention may be utilized to produce specifically tailored grafted polymer products that meet desired physical and chemical characteristics for a particular application (i.e., dispersant, viscosity index improver) while avoiding the necessity for the post-reaction processing (i.e., mechanical shearing) sometimes necessary in other methods of producing polymer products.
Another aspect of this invention is the production of a dispersant-viscosity index improver (DVII) by the aforementioned method. The graft copolymers produced by this method can have ADT dispersancy values of from about 2 to about 4, preferably from about 4 to about 8, more preferably from about 8 to about 16, and most preferably from about 16 to about 32 or higher.
In an alternate embodiment, this invention relates to a method for making a lubricating oil comprising: (a) providing (i) a lubricating base oil, and (ii) a dispersant viscosity index improver made by the method described above; and (b) mixing the dispersant viscosity index improver in the base oil.
In another alternate embodiment, this invention provides graft polymer products produced by the aforementioned methods. These graft polymer products differ from graft polymer products made from conventional methods in that they contain reduced levels of solvent, have greater seal compatibility, yet retain high ADT dispersancy values.